Conventionally, PZT (PbZrO3—PbTiO3-based solid solution) which is a ceramic material has been widely used as a piezoelectric material. Since PZT contains lead, however, as a piezoelectric material, polymeric piezoelectric materials (polymeric piezoelectric films) having low environmental load and being rich in flexibility are now being used.
Currently known polymeric piezoelectric materials are poled polymers represented by Nylon 11, polyvinyl fluoride, polyvinyl chloride, polyurea, polyvinylidene fluoride (β-type) (PVDF), vinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)) (75/25), and the like.
In recent years, attention has been given to using polymers with optical activity, such as polylactic acids, in addition to the above-described polymeric piezoelectric materials. Polylactic acid-type polymers are known to express piezoelectricity only by a mechanically, stretching operation.
The piezoelectricity of polymer crystals such as polylactic acids among the polymers with the optical activity are caused by permanent dipoles with C═O bonds that are present in a helical axis direction. In particular, the polylactic acid has the low volume fraction of side chains based on that of main chains and the high percentage of permanent dipoles per volume, so that it may be said that it is an ideal polymer among the polymers having helical chirality. The polylactic acid that expresses the piezoelectricity only by the stretching treatment does not require poling treatment and is known to have a piezoelectric modulus that does not decrease for several years.
As described above, since polylactic acid has various piezoelectric characteristics, polymeric piezoelectric materials using various polylactic acids have been reported (see, for example, Patent Documents 1    Patent Document 1 Japanese Patent Application Laid-Open (JP-A. No. H05-152638    Patent Document 2 JP-A No. 2005-213376    Patent Document 3 JP-A No. 2014-086703